I now realise that the PSPP update command reads data from the Master file
and reads data from the Transaction file and from this data the 'active'
file is manipulated to reflect the intended updates. The original data in
Master remains unaffected. At the end of the run nothing changes. To
retain the updates the active file must be saved.

The user manual is a little ambiguous in this regard. The first sentence
states "UPDATE updates a master file..." which is not quite correct, as the
master file is not updated. Then the first dot point refers to values
being replaced "in the new active file." I missed this on my first reading
of the manual. May I suggest the manual be changed to make it clear that
the master file is not actually updated and that the active file must be
saved to retain a permanent record of the updates